Dissertations / Theses on the topic 'Fatigue (Materials)'

A study has been undertaken of fatigue in glass fibre reinforced composites. Two matrix resins were tested; an isophthalic polyester and a polyurethane-vinyl-ester, which was designed to have superior properties, including toughness and resistance to hydrolytic attack. Three different types of glass fibre fabrics were used for reinforcement, a conventional woven roving and two novel stitch-bonded cloths. The resins and cloths were combined into eight lay-ups in order to consider the effects of matrix, cloth and lay-up on fatigue strength and lifetime. The fatigue study was extended to evaluate the micromechanisms that occur in these composites during fatigue and how damage accumulated throughout the sample lifetime. This involved measuring stiffness changes during fatigue cycling combined with microscopic study of the samples. The damage mechanisms that occurred were similar to those seen by previous authors on different materials and from this, it was concluded that the same mechanisms occur independent of material and lay-up but these parameters affect the point in the specimen lifetime at which the damage occurs. After the data had been obtained, two experimental models were compared against data obtained in the S-N and damage accumulation studies to evaluate whether existing models would predict the behaviour of these composites. It was found that modelling of the linear portion of the S-N curve was fairly accurate but the damage accumulation model was not suitable. The composites were also fatigue tested in various environments and compared against the results obtained in air. Distilled water, sea water and dilute HCl were chosen as being the most likely encountered in the service of these materials. It was found that distilled water and sea water have minimal effect on fatigue in these composites during the short lifetimes used in this study, but it is suggested that the effect would increase with lifetime. The dilute HCl acid also had a smaller than expected effect. This study was backed with various tests which studied methods of water transport into these materials and the effects of the environments on matrix and fibre properties. Finally, initial studies have been made into methods of fabricating these materials into composite tubes with the aim of studying their properties in torsion and possibly tension-torsion.

Fatigue is one of the critical design aspects with immense significance where thefatigue life of a material can be stated as the number of cycles that a componentcan withstand under a particular type of loading without failure. The design processhas to include fatigue analysis in order to predict failure due to fatigue. This helpsin maintenance and servicing of a component reducing the chance of failure duringoperation of the component. Increased efficiency of predictive maintenance improvesthe life of the component.This thesis aims to study the relationship between the experimental, analytical andnumerical solutions of two high strength aluminium alloys and one steel alloy fortheir life in aircraft applications covering the effects of geometrical irregularities. Italso aims to answer convergence between the numerical and the analytical methodwhen compared with each other. The simulations are carried out for three materialsamong many used in aircraft and industrial applications (Al 7050-T7451, Al 7075-T6 and AISI 4340 Steel) for a pre-defined set of geometries. The stress field andthe stress concentration factor variations are also studied to identify their effects onfatigue life.The results from this work forms a strong background for the future research alongside SAAB or any other industries using these materials for their structures to findout the failure or predicting it accurately. Also, integral structures can be analysedin detail using this thesis as a base.

Within the field of rapid prototyping a range of metal materials and production techniques have emerged. One field of application which has been addressed is for elevated temperature applications, namely die-casting. This thesis will investigate a range of rapid tooling materials available at the commencement of the work for use in aluminium pressure die casting. A series of experiments were conducted to answer the following research questions. (1) To what extent can rapid tooling materials resist thermal cycling and be used as a solution for aluminium pressure die casting? (2) If the thermal profile of an aluminium pressure die cast tool can be obtained, can it be simulated? (3) Can the thermal properties, failure mode, and life expectancy of rapid tooling metal materials be determined? (4) From the data obtained, is it possible to predict how other rapid tooling or like-materials would behave when subjected to thermal fatigue and can their suitability as a die casting tool material be determined?

The overall objective of this study is to develop methods for providing a statistical summary of material fatigue stress-life (S-N) data for engineering design purposes. Specific goals are: (1) Development of an analytical model for characterizing fatigue strength. This model would include: (a) a description of the trend of the data (e.g., the median curve through the data), (b) a description of the scatter of the data (e.g., the standard deviation of N as a function of S), and (c) the statistical distribution of N given S or S given N. (2) Development of an algorithm for constructing a design curve from the data. The curve should be on the safe side of the data and should reflect uncertainties in the physical process as well as statistical uncertainty associated with small sample sizes. (3) Development of a statistical model that can be applied in a structural reliability analysis in which all design factors are treated as random variables. Significant achievements are: (1) Demonstration, using representative fatigue data sets, that the bilinear model seems to provide a consistently adequate description of the trend of fatigue data. (2) Demonstration, using representative fatigue data sets, that the pure X error source model seems to provide a consistently adequate description of the uncertainties observed in heteroscedastic fatigue data. The pure X error source model is based on recognition of the uncertainties in local fatigue stress. (3) Development of a procedure for constructing a design curve using the tolerance limit concept developed by D. B. Owen. A more practical simplified or approximate Owen curve was shown to have a minimum loss of confidence level, relative to exact Owen theory, under fairly general conditions. (4) Recommendations for methods of developing a statistical model for reliability analysis. A comprehensive study of this issue was not pursued.

The global energy consumption is increasing and together with global warming from greenhouse gas emission, create the need for more environmental friendly energy production processes. Higher efficiency of biomass power plants can be achieved by increasing temperature and pressure in the boiler section and this would increase the generation of electricity along with the reduction in emission of greenhouse gases e.g. CO2. The power generation must also be flexible to be able to follow the demands of the energy market, this results in a need for cyclic operating conditions with alternating output and multiple start-ups and shut-downs. Because of the demands of flexibility, higher temperature and higher pressure in the boiler section of future biomass power plants, the demands on improved mechanical properties of the materials of these components are also increased. Properties like creep strength, thermomechanical fatigue resistance and high temperature corrosion resistance are critical for materials used in the next generation biomass power plants. Austenitic stainless steels are known to possess such good high temperature properties and are relatively cheap compared to the nickel-base alloys, which are already operating at high temperature cyclic conditions in other applications. The behaviour of austenitic stainless steels during these widened operating conditions are not yet fully understood. The aim of this licentiate thesis is to increase the knowledge of the mechanical behaviour at high temperature cyclic conditions for austenitic stainless steels. This is done by the use of thermomechanical fatigue- and creepfatigue testing at elevated temperatures. For safety reasons, the effect of prolonged service degradation is investigated by pre-ageing before mechanical testing. Microscopy is used to investigate the microstructural development and resulting damage behaviour of the austenitic stainless steels after testing. The results show that creep-fatigue interaction damage, creep damage and oxidation assisted cracking are present at high temperature cyclic conditions. In addition, simulated service degradation resulted in a detrimental embrittling effect due to the deterioration by the microstructural evolution.

Propagation of cracks in functionally graded materials (FGMs) under cyclic loading was investigated via experiments and finite element (FE) analysis. Alumina-epoxy composites with an interpenetrating-network structure and tailored spatial variation in composition were produced via a multi-step infiltration technique. Compressed polyurethane foam was infiltrated with alumina slip. After foam burn-out and sintering, epoxy was infiltrated into the porous alumina body. Non-graded specimens with a range of compositions were produced, and elastic properties and fatigue behaviour were characterised. An increase in crack propagation resistance under cyclic loading was quantified via a novel analytical approach. A simulation platform was developed with the commercial FE package ANSYS. Material gradient was applied via nodal temperature definitions. Stress intensity factors were calculated from nodal displacements near the crack-tip. Deflection criteria were compared and the local symmetry criterion provided the most accurate and efficient predictions. An automated mesh-redefinition algorithm enabled incremental simulation of crack propagation. Effects of gradient and crack-geometry parameters on crack-tip stresses were investigated, along with influences of crack-shape, crack-bridging, residual stresses and plasticity. The model provided predictions and data analysis for experimental specimens. Fatigue cracks in graded specimens deflected due to elastic property mismatch, concordant with FE predictions. In other FGMs, thermal or plastic properties may dominate deflection behaviour. Weaker step-interfaces influenced crack paths in some specimens; otherwise effects of toughness variation and gradient steps on crack path were negligible. Crack shape has an influence, but this is secondary to that of elastic gradient. Cracks in FGM specimens initially experienced increase in fatigue resistance with crack-extension followed by sudden decreases at step-interfaces. Bridging had a notable effect on crack propagation resistance but not on crack path. Similarly, crack paths did not differ between monotonic and cyclic loading, although crack-extension effects did. Recommendations for analysis and optimisation strategies for other FGM systems are given. Experimental characterization of FGMs is important, rather than relying on theoretical models. Opportunities for optimization of graded structures are limited by the properties of the constituent materials and resultant general crack deflection behaviour.

A modeling technique for simulating the fatigue behaviour of laminated composite materials with or without stress concentrations, called progressive fatigue damage modeling, is established. The model is capable of simulating the residual stiffness, residual strength and fatigue life of composite laminates with arbitrary geometry and stacking sequence under complicated fatigue loading conditions.
The model is an integration of three major components: stress analysis, failure analysis, and material property degradation rules. A three-dimensional, nonlinear, finite element technique is developed for the stress analysis. By using a large number of elements near the edge of the hole and at layer interfaces, the edge effect has been accounted for. Each element is considered to be an orthotropic material under multiaxial state of stress. Based on the three-dimensional state of stress of each element, different failure modes of unidirectional ply under multiaxial states of stress are detected by a set of fatigue failure criteria. An analytical technique, called the generalized residual material property degradation technique, is established to degrade the material properties of failed elements. This analytical technique removes the restriction of the application of failure criteria to limited applied stress ratios. Based on the model, a computer code is developed that simulates cycle-by-cycle behaviour of composite laminates under fatigue loading.
As the input for the model, the material properties (residual stiffness, residual strength and fatigue life) of unidirectional AS4/3501-6 graphite/epoxy material are fully characterized under tension and compression, for fiber and matrix directions, and under in-plane and out-of-plane shear in static and fatigue loading conditions. An extensive experimental program, by using standard experimental techniques, is performed for this purpose. Some of the existing standard testing methods are necessarily modified and improved. To validate the generalized residual material property degradation technique, fatigue behaviour of a 30-degrees off-axis specimen under uniaxial fatigue loading is simulated. The results of an experimental program conducted on 30-degrees off-axis specimens under uniaxial fatigue show a very good correlation with the analytical results. To evaluate the progressive fatigue damage model, fatigue behaviour of pin/bolt-loaded composite laminates is simulated as a very complicated example. The model is validated by conducting an experimental program on pin/bolt-loaded composite laminates and by experimental results from other authors. The comparison between the analytical results and the experiments shows the successful simulation capability of the model.

This investigation addresses the interaction mechanisms of time dependent material behavior and cyclic damage during fatigue loading of fiber reinforced composite laminates. A new term 'damorheology' has been coined to describe such physical behavior. The lamina has been chosen as the building block and a cross ply laminate configuration was the selected test case. The chosen material system is the Radel X/T65-42 thermoplastic composite by Amoco. The fatigue performance at the lamina level is represented by the dynamic stiffness, residual strength and fatigue life of unidirectional laminates. The time dependent behavior is represented at the lamina level by a Pseudo-Analog Mechanical model. The thermo-rheological characterization procedure combines mechanical (creep) and thermal (dynamic mechanical analysis) techniques.
Ph. D.

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Thesis (Doctorate)
IPEN/T
University of Manchester, England

Rubber components under cyclic loading conditions often are considered to have failed as a result of the stiffness changing to an amount that makes the part no longer useful. This thesis considers three distinctive but related aspect of the fatigue failure exhibited by rubber components. The first considers the reduction in stiffness that can result from a phenomenon known as cyclic stress relaxation. The second considers fatigue crack growth encountered resulting in potentially catastrophic failure. The final issue relates to the complex topography of the resulting fatigue fracture surfaces. Previous work has shown that the amount of relaxation observed from cycle to cycle is significantly greater than that expected from static relaxation tests alone. In this thesis the reduction in the stress attained on the second and successive loading cycles as compared with the stress attained on the first cycle in a stress strain cyclic test of fixed strain amplitude has been measured for elastomer test pieces and engineering components. Adopting the approach of Davies et al. (1996) the peak force, under cyclic testing to a specific maximum displacement, plotted against the number of cycles on logarithmic scales produces a straight line graph, whose slope correlates to the rate of cyclic stress relaxation per decade. Plotting the rate of stress relaxation per decade against the maximum average strain energy density attained in the cycle reduces the data measured in different deformation modes for both simple test pieces and components to a single curve. This approach allows the cyclic stress relaxation in a real component under any deformation to be predicted from simple laboratory tests (Asare et al., 2009). Earlier work (Busfield et al., 2005) has shown that a fracture mechanics approach can predict fatigue failure in rubber or elastomer components using a finite element analysis technique that calculates the strain energy release rate for cracks introduced into bonded rubber components. This thesis extends this previous work to examine real fatigue measurements made at both room temperature and 70±1ºC in both tension and shear using cylindrical rubber to metal bonded components. Dynamic testing of these components generated fatigue failures not only in the bulk of the component but also at the rubber to metal bond interface. The fatigue crack growth characteristics were measured independently using a pure shear test piece. Using this independent crack growth data and an accurate estimate for the initial flaw size allowed 3 the fatigue life to be calculated. The fracture mechanics approach predicted the crack growth rates accurately at both room temperature and 70±1ºC (Asare et al., 2011). Fatigue crack growth often results in rough fatigue crack surfaces. The rough fatigue crack surface is, in part, thought to result from anisotropy being developed at the front of a crack tip. This anisotropy in strength whereby the material is less strong in the direction that the material is stretched might allow the fatigue crack to grow in an unanticipated direction. It might also allow the crack front to split. Therefore the final part of this thesis examines how, once split, the strain energy release rate associated with growth of each split fatigue crack develops as the cracks extend in a pure shear crack growth test specimen. The aim being to understand how the extent of out of plane crack growth that results might allow a better understanding of the generation of particular crack tip roughness profiles. Using a method of extending one split crack at a time, whilst keeping a second split crack at a constant length, it has been possible to evaluate the initial strain energy release rates of split cracks of different configurations in a pure shear specimen. It was observed that, for a split crack in a pure shear specimen, the initial strain energy release rate available for crack growth depends on the precise location of the split crack. It is also clear that the tearing energy is shared evenly when the crack tip is split into two paths of equal length, but as one crack accelerates ahead it quickly increases in tearing energy and leaves the slower crack behind. It is thought that this phenomenon is responsible for a lot of the roughness observed on the resulting fracture surfaces.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2002.
Includes bibliographical references (p. 125-135).
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Fretting fatigue is defined as damage resulting from small magnitude (0.5-50 microns) displacement between contacting bodies where at least one of the bodies has an applied bulk stress. The applicability and limits of a fracture mechanics based life prediction is explored. Comparisons are made against highly controlled experiments and less controlled but more realistic experiments using a novel dovetail attachment fixture. Surface engineering approaches are examined from a mechanics perspective. Using a new tool, depth sensing indentation, the mechanical properties of an aluminum bronze coating are determined. Fretting fatigue experiments are performed on specimens coated with aluminum bronze and on specimens treated with low plasticity burnishing. Low plasticity burnishing is a new method of introducing beneficial compressive residual stresses without significant cold work at the surface. A mechanics based approach to the selection of palliatives is addressed.
by Brett P. Conner.
Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering; and, (Nav.E.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1999.
Includes bibliographical references (leaves 96-100).
by Gary W. Kirkpatrick.
S.M.
Nav.E.

The objective of this study is to develop crack growth analysis methods for functionally graded materials under mode I cyclic loading by using finite element technique. The study starts with the analysis of test specimens which are given in ASTM standard E399. The material properties of specimens are assumed to be changing along the thickness direction according to a presumed variation function used for the modeling of functionally graded materials. The results of the study reveal the influence of different material variation functions on the crack growth behavior. In the second part, the growth of an elliptical crack which is a common case in engineering applications is analyzed. First, mode I cycling loading is applied perpendicular to the crack plane and crack growth profiles for a certain number of cycles are obtained for homogeneous materials. Then, the code is extended for the analysis functionally graded materials. The material properties are assumed to vary as an exponential function along the major or minor axis direction of the crack. The results can be used to examine the crack profile and material constants&rsquo
influence for a certain number of cyclic loading.

Thanks to their lightness and versatility combined with excellent mechanical properties, composite materials underwent an increasing relevance in the last twenty years in many industrial fields, also for structural applications. This class of materials offer a wide range of advantages but it is still characterized by a quite high cost with respect to more traditional structural materials. One of the reasons is the lack of reliable design procedures, as well as of extensive experimental investigations in the literature providing clear and general information. This is particularly evident with reference to the behaviour of composite laminates and bonded joints under multiaxial fatigue loading, which often characterise in-service conditions. With the aim to acquire information on the multiaxial fatigue behaviour of unidirectional composites, a specimen configuration was first defined, suitable to characterise the matrix-dominated multiaxial fatigue behaviour of a composite lamina, which is of great importance for the damage evolution in laminates (chapter 2). Tubular specimens subjected to combined tension/torsion loading were identified as the best compromise between reliability of results, easiness of testing and possibility to obtain multiaxial stress states of interest. In the third chapter the results of an extensive experimental investigation on tubular specimens is presented. Tubes were fatigue tested with several values of the biaxiality (shear to transverse) ratio and load ratio (ratio between the minimum and the maximum fatigue loads). A strong influence of both parameters was found on the off-axis crack initiation and propagation phenomena, as well as on the damage mechanisms at the micro-scale. Uniaxial fatigue tests were then carried out on multidirectional flat laminates, designed to achieve local multiaxial stress states similar to those applied to the tubes by means of tension/torsion external loads. This activity, presented in the fourth chapter, revealed the equivalence between external (obtained by applying external loads in different directions) and internal (due to material anisotropy) multiaxial stress states. This represents a fundamental step for the extension of experimental results and predictive models to general loading conditions. The experimental activity on tubes provided information on the damage mechanisms at the micro-scale, responsible for fatigue failure of a unidirectional lamina. On the basis of these mechanisms a criterion for predicting crack initiation in a unidirectional lamina under multiaxial fatigue was developed by means of a multiscale approach (chapter 5). This criterion resulted in sound agreement with the new data on tubes, with data on flat unidirectional laminates from the literature and with crack initiation data on the off-axis layers of the laminates reported in chapter 4. A basic topic related to the analysis of multidirectional laminates is the stiffness degradation due to off-axis cracks in their plies. Dealing with such a topic, an analytical model was developed for predicting the stiffness of a laminate as a function of the crack density in its layers accounting for the interaction between cracks in different layers (chapter 6). In addition the model is capable of calculating the stress re-distribution due to the presence of cracks. This is fundamental for the development of a new procedure for predicting the fatigue crack density evolution in multidirectional laminates, presented in chapter 7. To this aim the experimental observations, the analytical models and criteria previously presented, combined with a statistical approach, have been used to predict the initiation and propagation of multiple cracks in a laminate. As a consequence, when this procedure is combined with the model presented in chapter 6, both the stiffness degradation and the stress re-distribution, useful for the estimation of the total fatigue life, can be predicted. When composite laminates are used as adherends in bonded joints the bonding surface represents a critical position for the onset of fatigue cracks. As a consequence an experimental investigation on the propagation of a bondline crack in composite bonded joints subjected to mixed mode I + II (opening + sliding) fatigue loading was carried out and presented in chapter 8. A criterion to predict the crack propagation rate under mixed mode loading was also developed, based on the damage mechanisms observed during the experimental campaign. Eventually, in Appendix A an analytical model for predicting the initiation of a fibre-matrix debond crack under biaxial static loads is presented. The model provides useful information on the influence of the main geometrical and interface parameters of the fibre-matrix interface strength.
Grazie alla loro leggerezza e versatilità combinate ad eccellenti proprietà meccaniche, i materiali compositi hanno acquisito un'importanza sempre maggiore negli ultimi vent'anni in molti settori industriali, anche per applicazioni strutturali. A fronte dei numerosi vantaggi offerti da questa classe di materiali vi è un costo che rimane ancora piuttosto elevato rispetto ai più tradizionali materiali da costruzione. Una delle ragioni è la mancanza di procedure di progettazione affidabili e riconosciute, nonché l'assenza, in letteratura, di estese caratterizzazioni sperimentali da cui acquisire informazioni di carattere generale. Ciò è particolarmente evidente in riferimento al comportamento a fatica multiassiale di lamine, laminati e giunzioni incollate in composito. Al fine di sopperire alla mancanza di informazioni sul comportamento a fatica multiassiale di lamine unidirezionali, nonché all'assenza di una procedura adeguata di test, è stata inizialmente definita una configurazione di provini adatta a caratterizzare la risposta matrix-dominated (particolarmente significativa per il danneggiamento a fatica di laminati) di materiali compositi unidirezionali (capitolo 2). Provini tubolari soggetti a carichi ciclici di trazione e torsione combinati sono stati identificati come il miglior compromesso tra affidabilità dei risultati, semplicità di testing e possibilità di ottenere condizioni multiassiali di interesse. Nel terzo capitolo sono riportati i risultati di un'estesa campagna sperimentale su tali provini tubolari in presenza di diversi rapporti di biassialità (tensione di taglio su tensione trasversale) e rapporti di ciclo (rapporto tra il minimo e il massimo carico di fatica). É stata riscontrata una notevole influenza di tali parametri sull'innesco e propagazione di cricche off-axis, nonché sui meccanismi di danneggiamento su scala microscopica. Sono poi stati testati a fatica uni-assiale dei laminai piani multi-direzionali progettati per avere condizioni di multiassialità locali simili a quelle ottenute sui provini tubolari tramite carichi esterni in diverse direzioni. L'attività, presentata al quarto capitolo, ha permesso di verificare l'equivalenza tra condizioni multiassiali ti tipo esterno (carichi in più direzioni) e interno (dovute all'anisotropia di lamine e laminati in composito). Ciò rappresenta uno step fondamentale per l'estensione di risultati sperimentali e modelli previsionali a condizioni di carico generiche. L'attività sperimentale sviluppata sui tubi ha fornito informazioni sui meccanismi di danneggiamento a livello microscopico che sono responsabili del cedimento a fatica della lamina unidirezionale. Sulla base di tali meccanismi è stato proposto un criterio per l'innesco di cricche a fatica multiassiale in lamine in composito basato su un approccio multiscala (capitolo 5). Il criterio è risultato in ottimo accordo con i nuovi dati sperimentali sui campioni tubolari, con dati disponibili in letteratura riguardanti lamine unidirezionali piane e con i dati ad innesco sugli strati off-axis dei laminati testati al capitolo 4. Parlando quindi di laminati multi-direzionali, un aspetto fondamentale è la diminuzione di rigidezza di questi ultimi dovuto all'innesco e propagazione di cricche multiple negli strati off-axis. A tal proposito è stato proposto un modello analitico in grado di legare la densità di cricche in ciascuno strato di un laminato alla diminuzione di rigidezza globale considerando anche l'interazione tra cricche presenti su strati diversi (capitolo 6). Tale modello fornisce anche le distribuzioni di tensione dovute alla presenza delle cricche stesse. Questo è un aspetto di fondamentale importanza per lo sviluppo di una procedura per prevedere l'evoluzione della densità di cricche in laminati multi-direzionali sollecitati a fatica, presentata al capitolo 7. A tale scopo le osservazioni sperimentali, i modelli analitici e i criteri sviluppati in precedenza, combinati ad un approccio di tipo statistico, vengono utilizzati per prevedere l'innesco e propagazione di cricche multiple in un laminato. Di conseguenza, combinata con il modello precedentemente illustrato, la procedura consente di prevedere sia la diminuzione di rigidezza di laminati sia la ridistribuzione delle tensioni per effetto del danneggiamento rappresentando quindi uno strumento utile anche alla stima della vita a fatica totale di un laminato. Quando i laminati in composito sono utilizzati come aderendi in giunzioni incollate, l'interfaccia di incollaggio rappresenta una zona particolarmente critica per l'innesco di cricche a fatica. Di conseguenza è stata analizzata sperimentalmente la propagazione di cricche in giunzioni incollate soggette a carichi ciclici di modo misto I + II (apertura + scorrimento). Ancora una volta i meccanismi osservati su scala microscopica sono stati utilizzati per la formulazione di un criterio damage-based per la previsione della velocità di propagazione di cricche in giunzioni incollate sollecitate in modo misto (capitolo 8). In fine, in Appendice A è presentato un modello analitico sviluppato per la previsione dell'innesco di una cricca di debonding tra fibra e matrice in condizioni di carico statico biassiale. Il modello è utile per trarre informazioni sull'influenza dei principali parametri geometrici e interfacciali sulla resistenza dell'interfaccia fibra-matrice.

Thesis (Ph. D.)--West Virginia University, 2000.
Title from document title page. Document formatted into pages; contains xiii, 183 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 180-183).

As current aerospace materials are subjected in service to increasingly onerous conditions of stress and temperature, the hazard of fatigue failure becomes more acute. Engineers utilise the methodology of fracture mechanics to estimate fatigue crack growth rates but fatigue crack initiation, which involves the interplay of many microprocesses, is only investigated empirically. The aim of this study was to investigate the fatigue damage accumulation mechanisms in the titanium alloy IMI 834 in order to develop a fundamental understanding of the controlling physical processes and the micromechanisms which occur at the dislocation level. Load controlled four point bend test specimens of IMI 834 were cyclically fatigued to failure with an R ratio of 0.1 over a range of maximum stress levels and the fatigue and fracture surfaces were examined by optical and scanning electron microscopy. The examination of cross-sectional foils prepared from the fatigue surface enabled the fatigue damage to be examined in the T.E.N. as a function of orientation and depth below the specimen surface. The distribution, orientation and type of slip bands were identified in the primary-a and the transformed-fJ grains, and their interaction with secondary phases, precipitates and grain boundaries was determined. The results show that fatigue damage accumulation in INI 834 occurs primarily on basal slip bands in the primary-a phase and on basal and prismatic slip bands in the transformed-fJ phase. The segregation of a-stabilising elements to the primary-a phase during alloy processing allows the formation of an ordered phase which increases the propensity for planar slip on the basal plane. A mechanism for fatigue crack initiation along this plane is proposed. In addition, the occurrence and identification of an interface phase is discussed in the light of current theories regarding this phase.

Thesis (M.S.)--University of Nevada, Reno, 2006.
"August, 2006." Includes bibliographical references (leaves 38-45). Library also has microfilm. Ann Arbor, Mich. : ProQuest Information and Learning Company, [2006]. 1 microfilm reel ; 35 mm. Online version available on the World Wide Web.

An investigation of the effect of R-ratio on the mode II fatigue delamination of AS4/3501-6 carbon/epoxy composites has been undertaken. Experiments have been performed on end notched cantilever beam specimens over a wide range of R-ratios (-l ≤R ≤0.50). The measured delamination growth rate data have been correlated with the mode II values of strain energy release rate range ∆G[formula omitted]), maximum strain energy release rate (G[formula omitted]) and stress intensity factor range (∆K[formula omitted]). The growth rate is dependent on the R-ratio over the range tested. For a constant level of ∆G[formula omitted], the crack growth rate decreases with increasing R-ratio. A similar trend is observed when the data is plotted as a function of G[formula omitted]. The effect of plotting the growth rate as a function of ∆K[formula omitted] is to produce an R-ratio dependence opposite to that obtained by either the ∆G[formula omitted] or G[formula omitted] approach. For a constant level of ∆K[formula omitted], the crack growth rate increases with increasing R-ratio. Master equations which completely characterize the fatigue behaviour as a function of ∆G[formula omitted] and ∆K[formula omitted] have been derived, based on the observation that the growth rate law exponent, n and constant, A are unique functions of R-ratio. Values for n are surprisingly large and increase with increasing R-ratio whereas values for A decrease with increasing R-ratio. The effect of time-at-load has been considered in an attempt to explain the existence of the R-ratio dependence of the growth rate. The correct trend can be established for the exponent, n but not for the constant, A. Friction between the crack faces, particularly at higher R-ratios, is proposed as a possible explanation for the observed anomaly. Further evidence of a frictional mechanism operating at higher R-ratios has been discovered through a postmortem fracture surface examination. Additional fractographic observations are presented over the entire range of R-ratios tested. In regions subjected to negative R-ratio cycling, there is no evidence of the characteristic mode II hackle features. Instead, loose rounded particles of matrix material are found. An extensive amount of hackling is observed in regions subjected to low positive R-ratio cycles. The extent of hackle damage visibly decreases in areas where higher levels of R-ratio are imposed. A correlation between the general fracture surface morphology and the fatigue data provides support for the hypothesis that energy for delamination is always available in sufficient quantity, and that growth is dependent on the stresses ahead of the crack tip being sufficiently high.
Applied Science, Faculty of
Materials Engineering, Department of
Graduate

This thesis describes a series of fretting fatigue experiments carried out under closely controlled conditions of partial slip. These experiments confirm the existence of a size effect whereby the fretting fatigue life of an aluminium alloy is shown to vary with contact size. The configuration chosen, of cylindrical fretting pads contacting a plane specimen is amenable to classical stress analysis and the surface tractions between the contacting bodies are derived. The effects of tension in the specimen, finite specimen thickness, differing elastic constants, and surface roughness are all investigated and incorporated into the analysis where appropriate. A technique is then developed to calculate stress intensity factors for plane cracks growing under the contact load at an arbitrary angle to the free surface. The analysis is then applied to the experimental results and three possible explanations for the size effect are proposed, based on statistical effects, crack arrest, and crack initiation. These are examined in the light of the experimental evidence and it is proposed that the variation of fatigue life with contact size is due to an increase in the amount of fretting damage above a threshold level for crack initiation. A composite parameter is chosen to characterise the severity of fretting conditions and this is shown to describe the experimental results accurately. Finally, the use of this parameter in design calculations is discussed.

Thesis (Ph.D.)--Polymer, Textile and Fiber Engineering, Georgia Institute of Technology, 2009.
Committee Chair: Realff, Mary L.; Committee Member: Graham, Samuel; Committee Member: Jacob, Karl I.; Committee Member: May, Gary; Committee Member: Shofner, Meisha.

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2009.
Committee Chair: Johnson, William Steven; Committee Member: Neu, Richard W.; Committee Member: Qu, Jianmin; Committee Member: Sanders, Thomas H.; Committee Member: Thadhani, Naresh N.

The purpose of this research is to examine structural durability of advanced composite materials under critical loading conditions, e.g., combined thermal and mechanical loading and shear fatigue loading. A thermal buckling model of a burnt column, either axially restrained or under an axial applied force was developed. It was predicted that for a column exposed to the high heat flux under simultaneous constant compressive load, the response of the column is the same as that of an imperfection column; the instability of the burnt column happens. Based on the simplified theoretical prediction, the post-fire compressive behavior of fiberglass reinforced vinyl-ester composite columns, which have been exposed to high heat flux for a certain time was investigated experimentally, the post-fire compressive strength, modulus and failure mode were determined. The integrity of the same column under constant compressive mechanical loading combined with heat flux exposure was examined using a specially designed mechanical loading fixture that mounted directly below a cone calorimeter. All specimens in the experiments exhibited compressive instability. The experimental results show a thermal bending moment exists and has a significant influence on the structural behavior, which verified the thermal buckling model. The trend of response between the deflection of the column and exposure time is similar to that predicted by the model. A new apparatus was developed to study the monotonic shear and cyclic-shear behavior of sandwich structures. Proof-of-concept experiments were performed using PVC foam core polymeric sandwich materials. Shear failure occurred by the extension of cracks parallel to the face-sheet/core interface, the shear modulus degraded with the growth of fatigue damage. Finite element analysis was conducted to determine stress distribution in the proposed specimen geometry used in the new technique. Details for a novel apparatus used for the fatigue testing of thin films and face sheets are also provided.

Thesis (Ph. D.)--University of Nevada, Reno, 2006.
"December, 2006." Includes bibliographical references (leaves 77-94). Online version available on the World Wide Web. Library also has microfilm. Ann Arbor, Mich. : ProQuest Information and Learning Company, [2006]. 1 microfilm reel ; 35 mm.

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2010.
Committee Chair: Dr. Steve Johnson; Committee Member: Dr. Jianmin Qu; Committee Member: Dr. Rick Neu. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1986.
MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE
Bibliography: leaves 185-199.
by Norman J. Marchand.
Sc.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1998.
Includes bibliographical references (p. 145-152).
This thesis presents theoretical, computational, and experimental approaches to the problem of fretting fatigue in materials systems relevant to aircraft components. The basic contact mechanics for fretting fatigue in a sphere-plane contact geometry are reviewed. Various elastic criteria for predicting fretting failure are discussed; selected fretting maps created from one of these - the modified Crossland criterion -- are presented. Fail/no-fail predictions based on these maps have verified trends observed in experimental work performed on Al 7075-T6 specimens. A three-dimensional fi­nite element model of sphere-plane fretting contact is reviewed. This model has been used to model elastic and elastoplastic fretting contact. The evolution of tangential loads coincident with plastic flow has been simulated, as well as the accumulation of equivalent plastic strains for these fretting conditions. This information may be used to predict the life of components subject to fretting contact high cycle fatigue (HCF) via a Coffin-Manson type relation. Design and construction of an apparatus for performing quantitative fretting experiments are described, and results of early tests performed on 7075-T6 aluminum alloys are presented. These experiments vali­date the proper operation of the experimental apparatus. Finally, basic principles of fracture mechanics and the limitations of applying traditional linear elastic fracture mechanics (LEFM) to fretting fatigue are discussed. The new crack analogue concept of Giannakopoulos et al. is reviewed as a means of uniting LEFM and fretting contact mechanics to achieve a life prediction scheme for components subject to HCF that is superior to the modified Goodman diagram approach currently employed by the US Air Force.
by Paul R. Birch.
S.M.

A lattice material is a cellular structure with a periodic arrangement of cells in either two or three dimensions. Lattice materials are attractive candidates for potential use in a broad range of applications, including battery electrodes, vibration insulators, ultra lightweight sandwich panels, and biomedical implants. This thesis focuses on the design of planar lattices for micro-architectured materials and medical devices. Strength of a lattice material degrades under cyclic loading conditions. In this thesis a computational method based on finite element analysis (FEM) is proposed to analyze and design lattice materials and structures for fatigue failure. A comparison with available experimental data contributes to the validity of the method. The effect of the unit cell's architecture on the fatigue resistance of lattice materials is investigated by considering square and hexagonal shapes of unit cells. A shape optimization methodology based on removing the stress concentration caused by the presence of geometrical discontinuities at the inner boundaries of the lattice cell walls is proposed to improve the fatigue resistance of planar lattice materials. The shape optimization method adapted for the fatigue design of a lattice is applied to design intravascular self-expandable characterized by a periodic arrangement of cells, against fatigue failure. In particular, the aim is to improve the fatigue resistance of Nitinol stent grafts with closed-cell, and to design a stent-like device functioning as a protection for an endovascular oxygenator. A parametric study was carried out to assess the effect of different geometrical parameters on the fatigue resistance and radial stiffness of the generated Nitinol stent lattices. Novel stent-like concepts are proposed to protect and guide the state-of-the art intravenous oxygenator developed by ALung Technologies Inc. (Pittsburgh, PA) in partnership with the University of Pittsburgh. The validity of the proposed concepts in protecting the oxygenator was tested in vitro. The structural behavior of the proposed conceptual designs was studied by using FEM, and the level of blood damage caused by catheter rotation is investigated through CFD analysis. Preliminary numerical and experimental observations suggest that the proposed design can put the oxygenator one step closer to the market.
Un matériau en treillis est une structure cellulaire avec une disposition périodique de cellules en deux ou en trois dimensions. Ces structures sont utilisées dans plusieurs applications, y compris les électrodes de la batterie, isolateurs de vibration, panneaux ultra légers en sandwich et implants biomédicaux. Cette thèse met l'accent sur la conception de réseaux plans pour des matériaux ayant une microarchitecture et pour les dispositifs médicaux. Dans plusieurs applications, la résistance d'un matériau en treillis se dégrade dans les conditions de chargement cycliques. Dans cette thèse une méthode numérique basée sur la mécanique de calcul est proposé afin d'analyser et de concevoir des matériaux et des structures en treillis pour prévenir toute rupture causée par fatigue. Une comparaison avec des données expérimentales contribue à la validité de la méthode. L'effet de l'architecture d'une cellule de cette unité sur la tenue en fatigue des matériaux en treillis est étudiée en tenant compte des formes carrées et hexagonales de cellules unitaires. En outre, une méthodologie d'optimisation de forme fondé sur l'élimination de la concentration du stress causé par la présence de discontinuités géométriques aux frontières intérieures des parois cellulaires en treillis est proposé pour améliorer la résistance à la fatigue des matériaux en treillis planaires. Plusieurs topologies de cellules augmentant la résistance à la fatigue sont proposées pour l'amélioration des matériaux et des structures caractérisées par un arrangement périodique de cellules. Cette méthode d'optimisation de forme adaptée pour la conception de fatigue d'un réseau de cellule est appliquée à la conception intravasculaire d'endoprothèses auto-expansibles et aussi à la conception d'un dispositif fonctionnant comme stent offrant une protection pour un oxygénateur endovasculaire.Une géométrie de la cellule avec une meilleure résistance à la fatigue est proposée pour un réseau planaire pour stent. Une étude paramétrique a été réalisée pour évaluer l'effet des différents paramètres géométriques sur la résistance à la fatigue et la raideur radiale des réseaux générés de stent. Plusieurs concepts nouveau empruntent du stent sont proposées pour protéger et guider un oxygénateur intraveineux mis au point par Technologies Inc. Alung (Pittsburgh, PA), en partenariat avec l'Université de Pittsburgh. La validité des concepts proposés assurant une protection de l'oxygénateur a été testée in vitro. Le comportement de la structure des conceptions proposées conceptuels a été étudié en utilisant la méthode des éléments finis tandis que et le niveau de dommages de sang causé par la rotation du cathéter a était évaluer à travers une modélisation numérique et dynamique des fluides. Les observations numériques et expérimentales suggèrent que la conception proposée mettrait l'oxygénateur un pas de plus vers le marché.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004.
Includes bibliographical references (p. 151-158).
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
The resistance of metals and alloys to fatigue crack initiation and propagation is known to be influenced significantly by grain size. Based on a wealth of experimental results obtained from microcrystalline metals, where the grain size is typically greater than 1 um, it is widely recognized that an increase in grain size generally results in a reduction in the fatigue endurance limit. On the other hand, a coarser grain structure can lead to an increased fatigue threshold stress intensity factor range, as well as a decrease in the rate of fatigue crack propagation. The relevance of these trends to ultra-fine-crystalline metals (grain size between 100 nm and 1000 nm) and nanocrystalline metals (grain size less than 100 nm) is relatively unknown. Such lack of understanding is primarily a consequence of the paucity of experimental data on the fatigue response of metals with very fine grains. In this work, the fatigue behavior of electrodeposited, fully dense, nanocrystalline pure Ni, with average and total range of grain sizes well below 100 nm, was examined. The fatigue response of nanocrystalline Ni was also compared with that of ultra-fine-crystalline and microcrystalline Ni wherever appropriate. It was found that grain refinement to the nanocrystalline regime generally leads to an increase in resistance to failure under stress-controlled fatigue whereas a deleterious effect was seen on the resistance to fatigue crack growth. To explore the generality of the above trends, similar experiments were performed on additional ultra-fine-crystalline material systems, produced using alternate processing techniques such as cryomilling and equal channel angular pressing.
(cont.) Contact fatigue behavior was also examined down to the nanocrystalline grain size regime. Friction and damage evolution was monitored as a function of the number of unidirectional sliding contact fatigue cycles introduced at the surface of several material systems. Critical experiments were performed to isolate the effects of grain size and material strength. Over the range of materials investigated, strength rather than grain size dominated the contact fatigue response, with substantial improvements in strength resulting in reduced damage accumulation, and a lower steady state friction coefficient. Conversely, grain size was found to govern the rate of crack growth under mechanical fatigue, with all other structural factors approximately held fixed. In addition, the cyclic deformation behavior of nanocrystalline materials was also investigated. Experiments designed to extract the strain response at a constant range of imposed cyclic stresses provided the first evidence of cyclic hardening in a nanocrystalline material. This behavior was observed over a broad range of loading conditions and fatigue frequencies.
by Timothy Hanlon.
Ph.D.